Fuel cell power systems convert a fuel and an oxidant to electricity. One type of fuel cell power system employs use of a proton exchange membrane (hereinafter “PEM”) to catalytically facilitate reaction of fuels (such as hydrogen) and oxidants (such as air or oxygen) to generate electricity. The PEM is a solid polymer electrolyte that facilitates transfer of protons from the anode to the cathode in each individual fuel cell of the stack of fuel cells normally deployed in a fuel cell power system.
In a typical fuel cell assembly (stack) within a fuel cell power system, individual fuel cells provide channels through which various reactants and cooling fluids flow. Fuel cell plates are typically designed with serpentine flow channels. Serpentine flow channels are desirable as they effectively distribute reactants over the active area of an operating fuel cell, thereby maximizing performance and stability. Movement of water from the channels to outlet manifolds of the fuel cell plates is caused by the flow of the reactants through the fuel cell. Drag forces pull the liquid water through the channels until the liquid water exits the fuel cell through the outlet manifold. However, when the fuel cell is operating at a lower power output, the velocity of the gas flow is too to low produce an effective drag force to transport the liquid water, and the liquid water accumulates in the flow channels.
A further limitation of utilizing gas flow drag forces to remove the liquid water is that the water encounters various surface irregularities with high or low surface energy or pinning points on the flow channel surfaces. Because the drag forces may not be strong enough to effectively transport the liquid water, the pinning points may cause the water to accumulate and pool, thereby stopping the water flow. Such pinning points are those commonly located where the channel outlets meet the fuel cell stack manifold.
Additionally, some current fuel cell assemblies utilize plates having hydrophilic surfaces. Water has been observed to form a film on the surface of the material that accumulates at the outlet of the flow channels and the perimeter of the plates. The water film can block the gas flow, which in turn reduces the driving force for removing liquid water and thus militates against the removal of the liquid water from the fuel cell stack. In the case of a fuel cell plate with a mildly hydrophobic surface, water has been observed to form large drops that protrude into the fuel cell stack outlet manifold blocking the exits of the channels of the fuel cell plates. The droplets are observed to remain at the plate edge until they can be intermittently removed by gas shear. The accumulation of water can cause gas flow blockages or flow imbalances that can have negative impacts on the performance of the stack.
It would be desirable to develop a fuel cell stack with an improved means for removing liquid water from fuel cell gas flow channels of the fuel cell stack, to minimize the accumulation of liquid water within the fuel cell stack.